Whisk together the milk and yeast in the bowl of a stand mixer (alternatively, use a large bowl if you are going to knead by hand). Let sit 10 minutes to dissolve; you will see a few bubbles but the mixture won't be foamy. Whisk together the granulated sugar, zest, eggs and 1 teaspoon salt in a small bowl.
Add the bread flour, whole-wheat flour, milk powder and the egg mixture to the yeast mixture and mix with a wooden spoon until a shaggy dough forms. Fit the bowl in place on a stand mixer fitted with the dough hook. Mix on medium speed until the dough comes together but is still slightly tough, about 3 minutes. Increase the speed to high and add 7 tablespoons of the butter, a tablespoon at a time (the dough will look broken after each addition, but it will eventually come together), and beat until the dough is smooth, elastic and very tacky but pulls away from the sides of the bowl, about 12 minutes. (This may seem like a long time, but the texture of the bread improves with a long knead time.) Alternatively, knead the dough on surface lightly dusted with bread flour, adding 1 tablespoon of the butter at a time, until smooth and elastic, about 15 minutes.
Cover with plastic wrap and let the dough rise in a warm, draft-free area until doubled in volume, 35 to 50 minutes. (Check it at 35 minutes: It should smell buttery and yeasty, and if it's doubled, it's done. If not, check again in 10 minutes.)
Meanwhile, whisk together the dark brown sugar, cinnamon and 1/4 teaspoon salt in a medium bowl until no lumps of sugar remain. Smash in 8 tablespoons of the remaining butter and stir until completely combined. Set aside until ready to assemble. Line a baking sheet with parchment.
Gently deflate the dough by lifting it up around the edges and letting it fall back into the bowl, turning the bowl and repeating if needed. Transfer to a clean work surface lightly dusted with bread flour. Roll into a 12-inch square, pushing the edges towards the center to make the straight sides. Spread the butter mixture over the dough in an even layer leaving a 1/4-inch border around the edges. Tightly roll into a log, pressing the sides occasionally to keeps them flush. Transfer to the prepared baking sheet and freeze for 20 minutes to firm up the dough.
Grease an 8-inch round cake pan with the remaining 1 tablespoon butter. Cut the dough log crosswise into 8 equal pieces and arrange, cut-side up, in the pan, gently shaping each cinnamon roll to make them fit in the pan. Let rise, uncovered, in a warm, draft-free area until doubled in volume and the rolls reach the top of the pan, 1 hour to 1 hour 30 minutes. Cover tightly with foil.
Fold an 18-inch piece of foil in half lengthwise, then fold it in half again lengthwise and finally once more in half lengthwise, making a 4 1/2-inch wide foil sling for the pan. Place a rack in a 6- or 8-quart Instant Pot® and fill with 2 cups water. Place the cinnamon rolls in the center of the sling and lower into the pot.
Follow the manufacturer's guide for locking the lid and preparing to cook. Set to pressure cook on the high-pressure setting for 40 minutes (see Cook's Note). After the pressure-cook cycle is complete, follow the manufacturer's guide for natural release. After 15 minutes, being careful of any remaining steam, unlock and remove the lid.
Meanwhile, whisk the confectioners' sugar, cream, vanilla and remaining 1/4 teaspoon salt in a small bowl until smooth and creamy, adding more cream if necessary.
Transfer the pan to a rack set inside a baking sheet, remove the foil and let cool 20 minutes in the pan. Invert onto a serving plate or platter. Drizzle the icing over the top and serve warm.
Settings may vary on your Instant Pot® depending on the model. Please refer to the manufacturer's guide.
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